Well logging



NOW 20 1945 D. SILVERMAN 2,389,24

WELL LOGGING Filed April 26, 1944 /52 @Q55 Q) l Y Patented Nov. 20, 1945' WELL LOGGING Daniel Silverman, Tulsa, Okla.,

or to Stanolind Oil and Gas Company, Tulsa, Okla., a corporation of Delaware Application April 26, 1944, Serial No. 532,757

8 Claims.

This invention pertains to the art of well signalling and has particular application to systems ior well logging during the course of drilling a well.

from the region Where the logging measurements of some earth characteristic are made, to the surlace of the earth. Early systems required the maintenance of a jointed system of insulated conductors extending up through the similarly Jointed string of drill pipe. Such systems were found to be extremely expensive to maintain, due to mechanical diiiiculties inherent in such a system in the presence of a moving stream of abrading well uid.

One means for applying a pulsating electric signal directly between the drill pipe or similar conductor extending into the well, by means of which a portion of the signal flows adjacent the surface or' the earth, producing voltage drops approximately radially away from the well,fhas been `described. in United States Patent 2,354,887 dated August 1, 1944, issued jointly to me and nobei't E. .bearon. This system involved the use or the so-called toroidal transformer. lhis provided not only an impedance matching device between the logging oscillator and a circuit including the conductor and a. portion of the earth .ML- Une of the main difficulties encountered in the it also provided a very strong mechanical system;

'i namely, one in which the drm pipe itself was unbroken and there was no section of insulated material in the drill pipe, through which the torque of this pipe must be transmitted.

The invention Vforming the subject matter of the present specification is an alternative system to that involving the toroidal transformer, and provides a well signalling system in which a drill pipe or similar conductor is employed which may be mechanically unbroken by any insulated section from top to bottom. 'l'his system provides a very easily constructed and cheap signalling system involving the advantages obtained by the use of the toroidal transformer Signalling system.

It is an object of this invention to provide a.

system of well signalling by means of which pnl-- sating electric currents'are transmitted through the earth between bottom and top of a. well, and in which the means of coupling the transmitter in such system to the earth circuit is electrically 65 highly eflcient, while providing at the same time ior a strong mechanical arrangement of drill pipe or the like, so that drilling can be accomplished during the time that the signalling is carried on. Another object ofthis invention is to provide a weil logging system capable for use during drilling operations, by means of which it is possible to log a characteristic of the formations shortly alter tney have been exposed by the drill and before there is substantial contamination by 'penetration of the drilling fluids. This system is characterized by the absence of any electrical conductors running from the logging region to the surface or the earth, and yet provides adequate signalling facilities so that it is possible to determine the earth characteristics while the well is being drilled although the well may be several thousand feet deep. Another object of this invention is to provide a signalling system by means of which electrical apparatus located near the drill bit may be turnedon or o at will from the surface 'of the ground. Still another object of this invention is to provide a coupling system between an earth circuit and a relatively high impedance detector circuit by means of which pulsating electric currents owing in the earth are caused to produce voltages in the detector system with relatively high efficiency, i. e. with fairly effective impedance matching. Other objects of this invention will be apparent upon perusal of the specification.

Certain drawings have been added to this specication as a part thereof and are to be read therewith, showing various embodiments of the invention. Since these drawings ane for illustrative purposes only, it is not to be considered that the invention is limited to the embodiments shown and described. In these drawings the same reference numerals refer to the same or a corresponding part. 1n these drawings:

Figure l represents in diagrammatic form a cross section of the earth penetrated by a well and one form of apparatus useful in carrying out the invention.

Figure 2 represents the electrical circuit of Figure 1 in somewhat greater detail.

Figure 3 represents an alternative signalling system, by means of which a signal applied at the surface of the ground is transmitted to log ging appar .itus located near the drill bit in a well penetratin f.: the earth.

Briefly what I propose to do is to use a string of drill pipe or the like as a conductor extending into the well from the surface, at least one section of which is covered with a layer of electrically trode which is thereby insulated from the conductor. A. voltage is applied between the electrode and the conductor. This voltage vis applied across the low turns winding of e. stepof coupling the relatively'low impedance earth circui comprising the conductor and the surrounding formations, and a high impedance electricel circuit disposed adjacent or within the con y ductorin the vicinity or the electrode.`

Such a signalling system is shown in Figure l. Here a well ll has been bored through the earth from the surface l2 bymeans of astring of drill .pipe I3. This well has been drilled in the customary manner :using the conventional rotary drilling system with a bit i4 at the lower end of the drill string I3. and employing drilling nuld which is circulated through the pipe and the annularspace between the pipe and the walls of the well. It is desired to los a eter-istie of the earth formations, for example, the resistance of the formations adjacent the well as the drilling proceeds. This should be accomplished as soon as possible after this so that there is a relatively small amount o penetration of water filtering out oi the well uids. The lower portion of the drill string is customarily made up of a drill collar yl5 which is composed of one or more sections of drill pipe with increased wall thickness, in order to give additional weight at the bit 'and to decrease the whipping action which would otherwise cause the hole to deviate from Va straight line. A considerable POrtion oi this lower section of drill pipe is covered with e wrapping or covering of insulating material lt. Any reasonably ilexlble insulating material may be employed for thlspurpose. Alternatively, relativelyl unflexible insulating material, such as Bakelite, may be made in the form of a cylindrical casing 1which is securely nxed in place on the outside of this drill collar l5., A major part ofA the outer surface of this insulation is eectlvely covered by an electrically conducting electrode il, This may be asolld cylindrical structure iirmly fastened about the insulation it, or y` be made up oi smaller conductors wound or woven together to give e ler-ee edective contact surface area. For example, this electrode may be ,made up of wire mesh or net, or can be made up of a 'spirally wrapped. metallic ribbon.

The chief characteristic of this electrode is that it has a large contact area. As will be discussed subsequently, when this electrode is used as part of a transmitting system, a diillerence of potentween the electrode and the conducting liquid incontact therewith should be quite small. This can be achieved only if the area of this electrode is quite large. 1 I have found that such an electrode should have an eiiective length .substan. tially greater than its diameter, andI prefer to use electrodes with a ratio of effective length to diameter of at least 5 and-more desirably 8 or more. There appears to be no upper limit to this ratio other than that imposed by the cost o1' preparation. By effective lengthy I mean the greatest distance between the ends of the electrode exposed to well Ailuid. The insulator II should project beyond the ends of the electrode both above and below. a. substantialdistance insulated material, over which isapplied 'an elecwhich can be. for example. of the order of the diameter of the electrode but which is preferably several times this amount at least. A conductor in a cable l! is securely attached to the surface of the electrode.

' Th'e actual logging apparatus itself is preferably located near the signalling electrode. One such arrangement of apparatus is shown in Pig- .ure 1. In this particular embodiment a tubular shell I9 with conical ends has been welded over one section of the drill pipe. The drill pipe itself has been plugged by two plugs 20. Suitable openings 2I have been out in the drill pipe above the top plug 28 and below the bottom plug 20 that the flow of drill mud through the drill l in this region is through the annular spac between the drill string and the tubular shell I9, as shown by the arrows. The space within the plugged-off portion of the drill pipe is occupied by the logging apparatus. In this figure a very simpliiied type of apparatus is shown. A section of the tubular shell I9 has been covered with a wide insulating sleeve 2l.- At or near the middle of this sleeve there is a ring electrode 23 of Y n conducting materiel. A conductor 24 is ccnnected to this ring electrode and pases into the center portion of the drill pipe through a hollow insulator 25. The conductor 2t is connected to an oscillator or alternator 26 which produces pulsating electric signals, for lexample a relatively constant voltage alteniating potential. The other terminal or" this oscillator or alternator 2t is connected to one side of the high turns winding of a step-down transformerl, the other side of which is grounded to the drill string G3. The voltage drop through the high turns winding of this transformer therefore depends upon the current owing between the drill string lil and the electrode 23 which in turn depends upon the resistance of the earth formations adjacent the electrode 23. The paths of current now to this electrode are shown roughlyby the dotlines 2d. The voltage across the high side of this transformer is therefore a direct indication of a characteristic of the formations, adjacent the well at this point.

The secondary of this step-down transformer, or the low turns winding. is connected between the drill string i3 and the conductor of cable it. so that the signal voltage proportional to the re slstance being logged is stepped down by means of the transformer, and the low voltage-high current signal is applied between the low resistance circuit made up of the drill pipe I3 and the electrode I1 on the outside of the insulation. This causes currents to now in one direction in the `drill pipe and through the earth in the other` direction to form aninilnite number of complete electric earth circui Portions of the lower end of the drill pipe. i. e.- the bit I4 and nearby parts, are also exposed to this flow of current and therefore there is a. shunt path between electrode I-'I 'and the drill string I3 which tends to reduce somewhat the elciency of transmission. How` ever, because of the'limited area ,of pipe and bit 4exposed below the electrode, the proportion of power lost by this means ls quite small. Some of the paths of current` flow'from the 5 distances from che axis pr conductor la. This` n to the drmstrmg n are shown by the lines in Figure 1. A portion-ot this currentnow from the earth to the drill string Il and-back occurs in the region Just-below the` suriya/ce of the earth J2.- This produces a drop of potentlallbe, tween two grounded electrodes at diilerent radial difference in potential can be picked up between electrodes 30 and 3I and detected (that is, amplifled, filtered if desired, and indicated) by the detector 32. These two electrodes 30 and 3 I, with their associated wiring, form one example of a circuit coupled to the earth near the head of the well, capable of causing current to flow in the earth upon application of pulsating electromotive force to this circuit. Any such means is useful either to detect or to transmit earth signals. It is desirable that the electrodes 30 and 3l be spaced a substantial distance apart, for ex ample of the order of 50 to several hundred feet, and electrode 30 may be either connected to ground, as shown, immersed in the drill uid at the well head, or connected directly by means of a brush to the drill string I3 itself. It is apparent that all such connections are substantially equivalent. l

The electrical system is shown in a slightly different form for ease in explanation in Figure 2. Here the electrodes are arranged as if they were buried at the surface of a conducting medium 33.

This conducting medium is, of course, the earthl represents the drill string I3 shown in Figure 1.

Part of this current ilow between electrodes I3 and Il occurs across the region of medium 33 in which electrodes 30 and 3l are placed, thereby producing a voltage drop between these electrodes which is amplied and indicated by the detector 32. Many such detecting arrangements have already' been shown in the prior .art and since any such detecting apparatus may be used, no detailed discussion of such device is given.

While in Figure 1 the electrode 23 for measuring the properties of the formations is shown mounted at some distance above the bit, it will be understood that this positioning of the electrade 23 is illustrative only. In many instances it will be desirable to measure the properties of the formations as soon as possible after they have been exposed by the bit, and in such cases the electrode 23 may be mounted just above the bit, for example, on the insulating sleeve I6 below the electrode Il. Electrode 23 could then be connected to the logging apparatus by an insulated lead passing through the interior of drill collar I5.

Further, it will be understood that the system of measuring the earths properties and subsequent modulation of the output of the oscillator 26 as shown in Figures 1 and 2 is only by way of example. Any other suitable type of modulation can be used, such as one where the measurement is made by currents of one frequency which then modulate currents of a different frequency for signalling.

It is apparent that the signalling system shown in Figures l and 2 can be employed to signal from the surface to the region near the drill bit, rather than in the opposite direction. In this case a source of alternating current is used instead of detector 32 and a detector of alternating current is used instead of the oscillator or alternator source 26. Such a system is shown in Figure 3.

In Figure 3 is shown a system in which the signalling is from the surface down to the bottom of the well. The particular application given is one in which the signalling apparatus is used to turn olf or on a logging apparatus located near the drill bit. This is only one example of many which could be given. In this ligure an alternator 35 or other-source of pulsating electric signals is applied between a region adjacent the well head of the well II and a point removed a considerable distance from the axis of the drill string I3. In Figure 3 this is accomplished by applying the voltage output of the source 35 between a brush 36 bearing: on ,the drill string I3,

and an electrode 3l buried at some distance from the well head. This produces current iiow through the earth between electrode 31 and drill string I3, some of the current flowing between electrode 31 and the bit I4 in the bottom of the apparatus, and more current flowing to the drill string in the region above this bit. For further discussion of this matter, reference is made to the specication in United States Patent 2,354,887 referred to above. Part of this current follows `a path shown by the dashed line 38 in Figure 3. I'his current path ends on the drill string I3 and produces a difference of potential between this drill string and an electrode 39 which is mounted on but insulated from the drill string I3. This electrode is of the type of electrode I1 shown in Figure 1, that is to say, this electrode is a conducting electrode which is eiectively a cylinder the length of which is at least 5 times its diameter and preferably 8 to 10 times this diameter or more. It is to be noted that the drill string I3 has welded to' it at this point a tubular shell 40 with conical ends. which provides a sealed annular space 4I between the outside of the drill pipe and the inside of this shell. In this space is mounted the electrical apparatus to be described. This electrical apparatus includes a stepup transformer 2l, the low turns winding of i which is connected between the insulated electrode 39 and the drill string I3, the high turns Winding of which is connected to a first lter 42 which is tuned to the frequency band of the out put of the alternator or oscillator 35. The ltered output of the iirst lter 42 is applied to a rst ampliiier 43 which may contain a demodulator. The output of this amplifier is applied to a modulator 44, which is also supplied with the output of oscillator 45. The modulated output of modulator 44 is applied to a second ampliiier 46, the output of which is applied in turn to a second iilter 41 tuned to the frequency band of the output of modulator 44. The output of this second filter is applied across the high turns winding of transformer 2l. The oscillator 45 is arranged to oscillate in a different frequency range than that of alternator or oscillator 35.

It is apparent from this description that the electrical apparatus shown acts as a repeater station, picking up a portion of the voltage output or signal due to the pulsating electromotive force of oscillator or alternator 35, and produces a signal varying in accordance therewith but in a different frequency band, between the electrode 39 and drill string I3. This causes current to ow in the earth between the drill string and theearth formations in a manner discussed in connection with Figure l. A part of this current ows to the drill bit I4 and the drill collar I5 below toroidal transformer 48. The construction and operation of this toroidal transformer is fully described in United States Patent No. 2,354,887 to which reference has been made. It is sumcient to say here that it is made up of a core of magnetic material surrounding the drill string, and a coil of insulated wire wrapped around the core so that currents owing through the drill string, i. e. through the core of the toroidal transformer, produce a voltage output across the winding wrapped around the core. Therefore a portion of the signals applied between electrode 39 and drill string is appear across the output leads 5S of this transformer. This output is amplied by an ampliier 5d which, although shown outside the drill string, is actually enclosed in a sealed chamber (not shown) within the walls of this drill string.- The output oi' this ampller is applied to the coil of a relay 5i. Therefore, when current ows through the earth due to a pulsating electr-emotive force applied between brush 3B and electrode 3l, the coil of'relay 5i is energized, closing the contacts thereof, which actuate the electrical logging apparatus .52. Since this electrical logging apparatus forms no pan of the invention per se and since several types'of such enclosed logging apparatus have been previously described, further description of this piece of equipment is unnecessary. A

WhileA I have described my invention in connection with certain embodiments thereof, it is understood that these are by way of illustration rather than b'y way of limitation, and that I do not mean to be restricted thereby, but only to the invention defined in the appended claims.

li claim: r '35 l.' Well signalling apparatus including an uninsulated conductor extending into said well, an electrode surrounding said conductor but insulated therefrom, the eective length of said electrode being substantially greater than its diameter, a step-down transformer the low turns winding of which is connected between said conductor and said electrode, a source of pulsating electromotive force, a detector of pulsating elecv tric signals, and means coupled to the earth near 40 the head of said well capable 'of 'causing current\ to dow in the earth upon application of pulsating electromotive force to said means, one of said source and said detector being connected to the high turns winding of said transformer and the other of said source and said detector being connected at the surface of the earth to said means.

2. Apparatus according to claim l inV which said electrode is made up of conducting meshend in which said insulation isa mapping of a thin layer of insulating material on said conductor.' extending a substantial distance above and below the ends of said electrode.

-3. Well signalling apparatus including an uninsulated conductor extending into said well, an 60- electrode surrounding said conductor, the ratio of 'eiective length of said electrode to its diameter being at least 5, a layer of insulation interposed between said conductor and said electrode, and extending a substantial distance above and 65 below the ends of said electrode, a step-down-A transformer thev low turns winding of which is connectedbetween said conductor and said electrode, a.- source of pulsating electromotive force connected 'to the high turns winding cf. said 70 transformer, and means coupled to the earth nearv the head of said well for detecting the flow oli-,pulsating 91385111; t0 said conductor and for producing a visuallndication of a'. characteristic of said detected current.

4. 1n well signalling apparatus including an uninsulated conductor extending into said well, the combination including an electrode surrounding said conductor, the ratio of effective length of said electrode to its diameter being at least 8, a layer of insulation interposed between said conductor and said electrode and extending a. substantial distance above and below the ends of said electrode, a step-down transformer the low turns winding of which is connected between said conductor and said electrode, a. source of pulsating electr-emotive force connected to the high turns winding of said transformer, and

' means coupled to the earth near the head of said well for detecting the dow of pulsating current to said conductor and for producing a visual indication of a. characteristic of said detected current.

5. Well logging apparatus including an 'uninsulated drill pipe extending into said well, said pipe being surrounded for the major portion of its length in said well by a conducting uid, a

thin Wrapping of insulating material surrounding said pipe near the lower end thereof, an electrode surrounding the major part of said insulating material, said electrode having a ratio of length to diameter of at least 8 and said material extending a substantial distance above and below the ends of said electrode, a step-down'coupling transformer the low turns winding of which is connected'between said pipe and said electrode, means for measuring an electrical characteristic associated with the earth formations surrounding said well, a source of alternating potential, means for varying a characteristic of said potential in proportion to the magnitude of said measurement, means for connecting said Varying potential across the high turns winding of said transformer, and means responsive to alternating cur-V rent ow in the earth adjacent the head of said well due to said source for producing a visual indication varying with the variations in said characteristic of said potential.

6. Well signalling apparatus including an uninsulatedconductor extending into said well, an electrode surrounding said conductor, the ratio of eiective length of said electrode to its diameter being at least 5, a layer of insulation interposed between said conductor and said electrode and extending a substantial distance above and below the ends of said electrode, a step-down transformer, the 10Wv turns Winding of which is connected between said conductor and said electrode, means coupled to the earth near the head of said well capable of causing current to iiow in the earth upon application of pulsating electromative force thereto, a source of pulsating electrotive force connected to'said means, means connected to the high turns winding of said transformer for detecting the iiow of pulsating current.

7. In well signalling apparatus including an uninsulated conductor extending-intosaid well,

" the combination including an electrode surrounding Said conductor, the ratio of effective length of said electrode to its diameter being at least 8, a layer of insulation interposed betweenv v said conductor and said electrode and extending 76Il nectedto said means, and means connected to the high turns winding of said transformer for detecting the tlow of pulsating current.

8. Well signalling apparatus including an uninsulated drill pipe extending into said well, said pipe being surrounded for the major portion of its length in said well by a conducting uid, a thin wrapping of insulating material surrounding said pipe near the lower end thereof, an electrode surrounding the major part of said insulating material, said electrode having a ratio of length to diameter of at least 8 and said material extending a substantial'distance above and below the ends of said electrode, a step-down coupling transformer the low turns winding of which is connected between said pipe and said electrode, means including a source of alternating potential coupled to the earth near the head of said well and causing current to now in the earth, means connected to the high turns winding of said transformer for detecting the flow of alter nating potential which is picked up by said elec' trade, an electrically operated switch, and means responsive to said detecting means for actuating said switch.

DANIEL SILVERMAN. 

